Considering it was initially a wierd experimental show that only required like half the effort of a normal action show (since the fight sequences were provided by Japan), I don't exactly blame the production for treating them as cheap and disposable.

Double so for every cast after the initial batch, who at least was able to ride the huge popularity wave to some extent. By the time of like "In Space" it was basically a dumping ground for Z list fodder they had hanging around the studio.